Field of the Invention
The present invention relates to a low-pressure casting apparatus and a low-pressure casting method using the same.
Description of the Related Art
A low-pressure casting apparatus for low-pressure casting has been known that is provided with a casting die and a holding furnace provided below the casting die for heating and holding molten metal (refer to Japanese Utility Model Laid-Open No. 1-89851, for example).
The casting die is provided inside with a cavity shaped to conform to the outer shape of a casting and a gate in communication with the cavity. The gate is connected to a stoke through a gate sleeve. A lower portion of the stoke is inserted into the molten metal heated and held inside the holding furnace.
In the low-pressure casting apparatus, the holding furnace includes a metal casing, a furnace body accommodated in the metal casing, and a refractory layer provided between the metal casing and the furnace body. The refractory layer is formed from a porous material with a pore structure, for example, and prevents dissipation of heat of the molten metal to the outside while keeping the molten metal at a predetermined temperature as it is used as a heat insulating material.
According to the low-pressure casting apparatus, a relatively low pressure gas such as compressed air is supplied into the holding furnace to apply pressure to the surface of the molten metal, so that the molten metal is pressed into the cavity through the stoke, the gate sleeve, and the gate. The molten metal inside the cavity is cooled down and solidified while being maintained in a pressurized state by the gas such as compressed air to thereby obtaining a casting.
In the low-pressure casting apparatus, when supplying the gas such as compressed air into the holding furnace, a volume of the space in the furnace body not occupied by the molten metal is estimated in advance with a casting model to estimate a molten metal surface height level to be obtained by pressure applied to the space. If the furnace body has a crack due to deterioration over time, however, the gas such as compressed air supplied into the holding furnace partially leaks through the crack to the refractory layer to make the pressure increase in the space slower than that of the casting model, thereby failing to obtain a necessary molten metal surface height level.
In order to solve the problem, feedback control of detecting the pressure for each shot and changing the gas supply amount can be considered in the low-pressure casting apparatus.
In the low-pressure casting apparatus, however, the actual pressure increase delays relative to the instructed gas pressure in the feedback control because compressive gas such as air is used to apply pressure to the molten metal, which is an inertial liquid. As a result, the delay is reflected in the feedback control, and then, an over shoot occurs where the actual pressure becomes higher than the instructed pressure in the following shot. This makes the molten metal surface wavy due to pressure fluctuation, leading to inconvenience such as casting failure.